New Zealand is conducting covert surveillance operations against some of its strongest trading partners and has obtained sophisticated malware to infect targeted computers and steal data, newly released documents reveal.

The country’s eavesdropping agency, Government Communications Security Bureau, or GCSB, is carrying out the surveillance across the Asia-Pacific region and beyond as part of its membership in the Five Eyes, a spying alliance that includes New Zealand as well as the United States, the United Kingdom, Canada, and Australia.

According to secret files from the National Security Agency, obtained by The Intercept from whistleblower Edward Snowden, GCSB is targeting about 20 different nations and territories in total and sharing the intercepted data with the NSA. A top-secret document dated from April 2013 notes that the New Zealand agency “provides [the NSA with] collection on China, Japanese/North Korean/Vietnamese/South American diplomatic communications, South Pacific Island nations, Pakistan, India, Iran, and Antarctica.”

Aside from eavesdropping on communications through traditional interception methods, such as by capturing signals as they are passing between satellites or phone cables, the New Zealand agency has also become directly involved in more aggressive methods of spying and cyberwar.

The newly revealed documents show that it has obtained a malware tool that is part of a platform named WARRIORPRIDE, used by the NSA and other Five Eyes agencies to hack into computers and smartphones, infect them with a bug, and then steal data. The documents note that GCSB “has a WARRIORPRIDE capability that can collect against an ASEAN target.” ASEAN, or Association of Southeast Asian Nations, may be a reference to New Zealand’s operations targeting Vietnam.

The surveillance being conducted by the GCSB shines light on a secret variant of New Zealand’s foreign policy that contrasts with its official public foreign policy.

Vietnam, for instance, has friendly relations with New Zealand and is a growing trading partner. The New Zealand government describes its relationship with Vietnam as having “flourished in the last 15 years.” The country poses no security or terrorist threat to New Zealand, the traditional explanation for GCSB operations given to the public. Yet its government is still on the GCSB spying list and its diplomatic communications have been eavsedropped on, likely in violation of the 1961 Vienna Convention of Diplomatic Relations, an international treaty ratified by New Zealand that says diplomats’ correspondence is “inviolable.”

Other targets, such as Japan and India, are publicly lauded as close and valued New Zealand partners. The New Zealand government says that Japan is “a major bilateral and regional partner of New Zealand” and describes having “a long-standing and warm relationship” with India. But each of these countries, too, is still being targeted for surveillance.

Another operation that sits uneasily with New Zealand’s official policy is GCSB’s monitoring of nations working in Antarctica. New Zealand is a signatory to the Antarctic Treaty; the country’s trade ministry says “involvement in Antarctica offers New Zealand the opportunity to play a constructive and influential role in a region … which is managed according to principles of international cooperation.”

The Antarctic eavesdropping takes place from New Zealand’s Waihopai base, a surveillance station that can intercept Antarctic satellite links. More than 20 nations have year-round scientific research bases in Antarctica, most of which use the same few satellite links to communicate: Intelsat, Inmarsat and Iridium phones.

A more predictable-sounding target is Iran, which has been a major target of American and British intelligence agencies for decades. However, in public, New Zealand has adopted a different long-term foreign policy with respect to Iran than has the United States and the United Kingdom.

Iran is a valuable export market for New Zealand agricultural produce and New Zealand has not joined in sanctions or confrontation with the Tehran government. But the GCSB has carried out secret intelligence operations, monitoring Iran presumably on behalf of the Five Eyes allies.

Similarly, China, which is New Zealand’s top export destination for goods and services, has also been targeted. In 2012, John Key launched a strategy to bolster New Zealand’s relationship with China and last year he praised the “the strength of the bilateral relationship between our two countries” during a speech in Beijing.

GCSB declined to comment on the latest revelations. In a statement issued to The Intercept and the New Zealand Herald late Monday, the agency’s acting director, Una Jagose, said: “The GCSB exists to protect New Zealand and New Zealanders. We have a foreign intelligence mandate. We don’t comment on speculation about matters that may or may not be operational. Everything we do is explicitly authorised and subject to independent oversight.”

A spokesman for prime minister Key said the government would not respond to “claims made from documents stolen by Edward Snowden.” He added: “New Zealand’s intelligence agencies have been, and continue to be, a significant contributor to our national security and the security of New Zealanders at home and abroad.”

NSA spokeswoman Vanee’ Vines said in a statement that the agency would not comment “on specific, alleged foreign intelligence activities.” Vines added: “The National Security Agency works with foreign partners to address a wide array of serious threats, including terrorist plots, the proliferation of weapons of mass destruction, and foreign aggression. NSA’s activities with foreign partners comply with U.S. laws and the applicable laws under which our partners operate.”